CHICAGO (REUTERS) - A violent weather system packing severe thunderstorms, damaging winds and possibly strong tornadoes hit parts of the upper Midwest on Wednesday and may soon batter Chicago, the nation's third-largest city, the National Weather Service said.

Some 12 million people in four states live in the area at high risk of severe storms that include baseball-size hail, possible strong tornadoes and heavy rainfall that could result in flash flooding, the agency said.

No tornado warnings had been issued in the high risk area by mid-afternoon on Wednesday, but the service predicted a "potentially very dangerous situation" into the evening through eastern Iowa, northern Illinois and Indiana and north-west Ohio.

Thunderstorms along a fast-moving squall line that could produce embedded tornadoes and damaging winds of up to 121km per hour are expected, according to the weather service.

"There is a tornado threat in the short term through 6pm to 7pm (central time), and damaging wind and hail," for the Chicago area, said Mr Greg Carbin, meteorologist for the Storm Prediction Center of the National Weather Service in Norman, Oklahoma.

The Chicago White Sox baseball game scheduled for Wednesday evening with the Toronto Blue Jays was postponed because of the threat of severe weather.

More than 20,000 fans were expected to converge on downtown Chicago at the time of the severe weather threat for the Stanley Cup hockey finals game between the Chicago Blackhawks and Boston Bruins at the United Center indoor arena.

Severe storms are also possible for a portion of the Mid-Atlantic and South-east, according to the service.